"In a couple of months it has gone from zero to more than 2 million users."

And he foresaw a bright future for Morpheus, and rivals such as Kazaa Media Desktop, as next generation applications continue to take users from Napster, which shut song swapping services in July after failing to meet court demands over blocking trade in copyrighted music.

Legal advantage

Legally, technical innovations at newcomers such as Morpheus limit their vulnerability to the kind of court action which forced the Napster suspension, Mr Sinnreich said.

"The Napster ruling centred on its status as an information service," he said. "The likes of Morpheus rely on matching offers and requests."

The recording Industry Association of America, which has spearheaded the fight against Napster, may need to launch further lawsuits to protect musicians' royalties, a JMM spokesman said.

"The RIAA's battle is not over," the spokesman said. "The Napster alternatives have surpassed Napster's user base and their momentum is increasing."

New markets?

Also, the software used by newcomers also represents a significant advance over that offered by many previous song-swapping ventures, Mr Sinnreich added.

"It did look as if the likes of Bodetella and Bearshare would take over, but they are way down the league now."

And the sites may benefit from the growth of file swapping services in other areas, such as add-on programmes for Photoshop, or Microsoft Word.

"It remains to be seen whether the likes of Morpheus will offer swapping across many areas, or whether file swapping services for say Photoshop plug-ins will be dealt with by specialists," Mr Sinnreich said.

Napster hopes

Overall, traffic to new generation song swapping services surged by 492% between June and August, JMM said.

Napster, with 5.5 million users, still attracted much more traffic than Morpheus in August.

But, with song swapping services shut, Napster's traffic was based around subsidiary services such as chatrooms.

"You have lots of people who have Napster as their default when opening MP3 files, so that as soon as they go to open the file, they go to Napster to do it," Mr Sinnreich said.

Napster, which claimed 10.5 million unique users in March, when court clampdowns began to bite, is set later this year to launch a fee-paying service, allowing it to meet royalty demands.